Accurate wireless temperature measurements using passive SAW sensors and a frequency modulation interrogation approach

Transcription

1 Accurate wireless temperature measurements using passive SAW sensors and a frequency modulation interrogation approach Sylvain Ballandras 1,2, Gilles Martin 1, Jean-Michel Friedt 2, Christophe Droit 2, Bruno François 1 1 FEMTO-ST, UMR 6174 CNRS-UFC-ENSMM-UTBM, Besançon France 2 SENSeOR SAS, Sophia-Antipolis, TEMIS Besançon, France

2 Forewords Resonator-based sensors Typical feature of a SAW resonator Practical configurations Summary Interrogation system Basic architectures A software-controlled general purpose platform Accuracy issues «Phase-locking» approaches Ultimate resolution Active oscillator-based measurement system Conclusion

3 Wireless interrogation of passive sensors First paper by X.Bao, Burkhard, Varadans', describes the basic idea in 1987 Siemens engaged work in that field, yielding first patents in 1995 (source Transense enters the competition in 2000 and propose advanced strategies for interrogating resonator-based sensor for pressure and torque (2001, source CTR, SENSeOR, sentec-elektronik, RSSI, Sengenuity, Sensor Technology and many other actors are now contributing to the activity

4 Two basic approaches Resonators (IIF) Reflective delay lines : Tags (FIR) (Courtesy of SENSeOR) (Courtesy of V. Plesski, GVR)

5 SAW resonators Single-port resonator Two-port resonator

6 Various SAWR designs Tapered synchronous resonator Hiccup resonator Synchronous resonators operate at the edge of the stopband Tapering the transducer reduces lateral mode contributions Asynchronous resonators (a/p or p) force resonance within the stopband Adding an extra λ/4 propagation path yields middle-of-the-band resonance for non directive resonators Combining all these features yields resonator optimization

7 SAW Resonator temperature sensor Sensor architectures 2 SAW on Chip 2 chips in a package Typical response F [MHz] f1 f T [ C] Sensor Characterization - Electrical admittance - Frequency/temperature laws (SENSeOR products)

8 Temperature-pressure sensor SAW temperature-pressure sensor SAW in ISM band : < f < MHz Pressure range : 0 20 Bars controlled by the membrane dimensions

9 SENSeOR Wireless system

10 Electronics bloc diagram A software-controlled system allowing for various interrogation strategy in the 434 MHz centered ISM band Micro-controllerr : AduC 7026, ARM7-core-based technology

11 Basic scanning operation V Réponse SAW-tag t Excitation Detection Straighten out Integration Thresholding 128-point frequency scan chronogram

12 Improving the accuracy The presented principle allows for temperature measurement by simply detecting the curve maximas using a fix scanning comb approach Advantage : Process very easy to implement Flaws : Inaccuracy due to the lack of coincidence of the measured max and the actual resonance peak Delay of spectrum scan : 128 (Q/π) τ~ 5,76ms A first improvement approach : fitting the max by a quadratic function and defining the actual max value solving a second degree polynomia

13 Parabolic fit : principle Fundaments SAW conductance near the resonance well represneted by a Lorentzian law A parabolic function reliably fits the conductance max The fit process can be fastly achieved using integer-based coding Computation needs a minimum calculation ressources

14 Application Application of the 3-point method to a 2-resonator temperature sensor

15 Tracking mode Rejected measurement Accepted frequency measurement

16 Stability Accuracy 5.76 ms for intialization 60 µs/point 360µs for 2 resonances An accuracy of 100 Hz is achieved for only 1 measurement The stability increases along measurement delay This reduces to 3 Hz when averaging 1000 samples

17 Frequency modulation strategy Modulation interrogation : Detection of the frequency modulation change from ω to 2ω at SAW resonance

18 Scanning the contribution at modulation frequency ω Efficiency of the approach depends on the amplitude and frequency of the modulation

19 Exploitation of the approach for phase-locking

20 Stability in wireless and wired configurations

21 Powered solutions Test-Oscillator Frequency counter Wireless protocol Zigbee transmitter Frequency reference Computer receiving data Zigbee receptor

22 Colpitts-based oscillator Ultimate accuracy given by the oscillator stability Phase noise below 150 dbc/hz Short term stability over 1 s yields sub Hz resolution, hence µkrange temperature accuracy

23 FPGA-based Counter HP frequency counter stability (53131A) FPGA-based frequency counter stability

24 Final implementation Oscillator + FPGA counter + Zigbee emitter Zigbee Receiver Interrogation distance > 30 m have been tested, 100 m achievable in theory

25 Conclusions Development of new strategies for wireless interrogation Using a software-based electronics allows for implementing numerous approaches Fix-comb approaches allow for accuracy in the 500 Hz at 434 MHz 3-point approaches yields improved frequency resolution (100 Hz and less when averaging) as well as the system passband (measurement delay ~200µs for a 2-resonator sensor) Phase-locking has been developed for reaching ultimate sensitivity provided the sensor is continuously interrogeable but needs longer delays Wired-powered solutions allows very large distance with coding

26 Perspectives Applications challenges In-motion measurements Reaching such accuracy for sensors fixed to moving parts with very high linear/rotation velocities Large band-width operation For monitoring fast processes such as stress evolution at frequencies above 5 khz Ultimate operation conditions Maintaining the obtained resolution when the reader faces extreme temperature/vibration/magnetic environments